<html>
    <head>
        <style type="text/css">
            .box {
                padding: 10px;
                padding-bottom: 0;
                padding-right: 0;
                display: flex;
                flex-direction: column;
                height: 800px;
                width: calc(100vw - 20px);
                background-color: #000;
            }
            .content {
                display: flex;
                margin-bottom: 10px;
                flex: 1
            }
            .b {
                flex: 1;
                margin-right: 10px;
                background-color: yellow;
            }
        </style>
    </head>
    <body>
        <div class="box">
            <div class="content">
                <div class="b">1</div>
                <div class="b">2</div>
                <div class="b">3</div>
            </div>
            <div class="content">
                <div class="b">4</div>
                <div class="b">5</div>
                <div class="b">6</div>
            </div>
            <div class="content">
                <div class="b">7</div>
                <div class="b">8</div>
                <div class="b">9</div>
            </div>
            <div>
    </body>
</html>

